Vertical cavity surface emitting devices like Vertical cavity surface emitting lasers (VCSELs), vertical external cavity surface emitting lasers (VECSELs), vertical cavity semiconductor optical amplifiers (VCSOAs), etc. present an important class of photonic devices. Normally, these devices incorporate an active region and one, or two distributed Bragg reflectors (DBRs). Depending on the emission wavelength, there are situations when the best performance of the active region and of the DBRs may be achieved using different semiconductor materials. One example are vertical cavity devices emitting in the 1200 nm-2300 nm band that make use of InP based active regions. Currently, InAlGaAs multi-quantum wells (MQWs) grown on InP substrates demonstrate best results. Besides high gain active cavity material in the full temperature operation range, distributed Bragg reflectors (DBRs) with very good optical and thermal properties are important for reaching high VCSEL performance. Unfortunately, distributed Bragg reflectors (DBRs) based on InAlGaAs/InP have inferior optical and thermal properties compared with AlGaAs/GaAs-based DBRs that can be grown lattice matched to GaAs substrates. (See Table 1).
TABLE 1Optical and thermal properties of different DBR material systemsIndexNo ofTotalContrastpairsStopbandthick-ThermalΔn =for 99.9%Width,ness,conductivity,DBR Materials(nH − nL)reflectivitynmμmW/cm KInAlGaAs/InP0.187635412.30.045AlGaAsSb/0.49271275.20.059AlAsSbAlGaAs/GaAs0.5231584.80.650SiO2/TiO20.7794363.30.015
AlGaAsSb DBRs lattice matched to InP have an index contrast between high-index (nH) and low index (nL) quarter-wavelength layers comparable to that of AlGaAs/GaAs-based DBRs but with one order of magnitude lower thermal conductivity. Dielectric DBRs, like SiO2/TiO2 have very good optical properties, but thermal conductivity is quite low. One solution to this problem is to use wafer fusion for bonding AlGaAs/GaAs DBRs on both sides of the InP/InAlGaAs based active cavity.
The U.S. Pat. No. 6,277,696 B1 describes the steps of surface emitting laser fabrication using two wafer bonded mirrors in the following way:                1) growing two AlGaAs/GaAs DBR mirrors on GaAs substrates and one InAlGaAs/InP active cavity on a InP substrate;        2) fusing one DBR mirror to the active cavity followed by InP substrate selective etch and fusing the second DBR mirror to the active cavity followed by GaAs substrate selective etch.        
These elements have been applied in the fabrication of different types of vertical cavity photonics devices as described in the following publications:    Single fused VCSELs based on one AlGaAs/GaAs DBR fused to InP-based active and DBR: S. Rapp, F. Solomonson, K. Streubel, S. Mogg, F. Wennekes, J. Bentell, M. Hammar “All-Epitaxial Single-Fused 1.55 um Vertical Cavity Laser based on an InP Bragg Reflector” Jpn. J. Appl. Phys., Vol. 38, pp. 1261-1264, 1999;    Double fused fixed wavelength VCSELs: V. Jayaraman et al., “High power 1320-nm wafer-bonded VCSELs with tunnel junctions”, IEEE Photonics Technology Letters, vol. 15, pp. 1495-1497, 2003;    Tunable VCSELs: A. Syrbu et all, “1.55 um optically pumped wafer-fused tunable VCSELs with 32-nm tuning range”, IEEE Photon. Technol. Lett., vol. 16, pp. 1991-1993, 2004;    Fixed wavelength optical amplifiers: E. S. Bjorlin, B. Riou, P. Abraham, J. Piprek, Y.-J. Chiu, K. A. Black, A. Keating, J. E. Bowers, “Long-wavelength vertical-cavity semiconductor optical amplifiers”, IEEE J. Quantum Electron., vol. 37, pp. 274-281, 2001”;    Tunable optical amplifiers: G. D. Cole, E. S. Bjorlin, C. S. Wang, N. C. MacDonald, J. E. Bowers, “Widely tunable bottom-emitting vertical cavity SOAs,” IEEE Photonics Technology Letters, 17, pp. 390-407, 2005.
This conventional method has produced a very good long-wavelength VCSEL performance. One example can be found in A. Mircea et al “Cavity Mode—Gain Peak Tradeoff for 1320-nm Wafer-Fused VCSELs With 3-mW Single-Mode Emission Power and 10-Gb/s Modulation Speed Up to 70° C.”, Phot. Techn. Lett. V. 19, issue 2, 2007, pp. 121-123.
Although VCSELs obtained by this method offer improved VCSEL performance compared to VCSELs with non-fused mirrors described in Table 1, the approach according to the U.S. Pat. No. 6,277,696 B1 has one important drawback that results from fusing of active cavity and the first DBR that are placed on different substrates, correspondingly on InP and GaAs, that have quite different values of lattice parameters and thermal expansion coefficients (See Table 2).
TABLE 2Lattice parameters and thermal expansion coefficients of GaAs and InPSubstrate materialLattice parameter, ÅThermal expansion Å/K, 106GaAs5.65355.8InP5.868754.8
In order to obtain an acceptable bonding strength, the wafer fusion is normally performed at 600° C. At this temperature very strong covalent bonds are formed between GaAs-based and InP-based wafers. During cooling down a considerable amount of strain is built-in the fused wafers stack as a result of different thermal expansion coefficients of initial wafers. At room temperature this ends-up in a substantial bending (elastic deformation) of the fused wafers stack with a radius of curvature of about 0.6 m (when the thickness of GaAs- and InP-based wafers is about 350 μm) (See FIG. 1).
After selective etching of the InP substrate 3 the fused wafer consisting of an InAlGaAs/InP active cavity 4 and an AlGaAs/GaAs reflector 2 on a GaAs substrate 1 is becoming planar again (See FIG. 2) because the GaAs-substrate with AlGaAs/GaAs on top, which is about 100 times thicker than the thickness of the active multi-layers returns to its initial planar state. As the result of this, the InP-based active cavity undergoes a lateral compressive strain to a point so that it laterally shrinks. The value of the shrinking, which represents a lateral material displacement in the active region is in the range of 3-7 μm per centimeter. This shrinking effect induces a problem for further processing steps that need a precise alignment with the electrical/optical aperture position defined in the active region before the fusion. Another important drawback consists in the formation of defects in the active region as a result of this high compressive strain. These defects represent a risk for device degradation during long-term operation.
It is the object of the present invention to provide an improved method of forming an optoelectronic device by high-temperature wafer fusion of wafers with different thermal expansion coefficients, wherein the build-up of strain and a lateral material displacement and a formation of defects in the active region related to it is reduced, and a corresponding semiconductor device.